Possible memory leak?

wkbrad nginx-forum at forum.nginx.org
Thu Mar 21 15:29:04 UTC 2019

Thanks again Maxim!  You're really providing some valuable insights for me.

> This is because mmap()-based individual allocations imply additional
> overhead. Using mmapAnd this is are costly.

That's what I figured might be going on.  I assume there are also some
negative impacts on performance with accessing memory that is allocated via
mmap.  Is that right?

> On FreeBSD you'll see that _virtual_ memory usage grows on reload,
> but the actual memory is returned to the system and can be used by
> other processes. 

Actually, I see an increase in virtual and resident.  Here are my tests from
service nginx restart; sleep 10; echo; ps aux|grep -v grep|egrep

root  923   0.0  5.4 121840 110368  -  Ss   08:10    0:00.00 nginx: master
process /usr/local/
www   924   0.0  5.4 121840 110372  -  S    08:10    0:00.01 nginx: worker
process (nginx)
service nginx reload; sleep 10; echo; ps aux|grep -v grep|egrep 'RSS|nginx'

root  923   0.0 10.2 219828 209388  -  Ss   08:10    0:00.44 nginx: master
process /usr/local/
www   940   0.0 10.2 219828 209384  -  S    08:10    0:00.01 nginx: worker
process (nginx)

Those tests look exactly like the Linux tests and it's definitely not
released back to the system.

I think a big part of this is the 2 heaps that Anoop found.  Nginx seems to
be using those 2 heaps in a round robin way when it reloads.  It looks like
it's doing this.
Startup: 1st heap is created
1st Reload: 2nd heap is created
2nd Reload: 1st heap is cleared and used but 2nd heap stays in memory.

So on the 2nd reload is does indeed clear that memory for the heap it is
using.  Is there any way for us to manually clear the unused heap?

Even in the case of my test VM using the adjusted malloc, it still creates
the second heap and does not clear it.  It's just not putting much in the
heap in my test VM.

You may be asking why I'm even looking into this and I've had a lot of push
back and questions as to why I'm doing this.  My point is this.  Even if
Nginx on my server is only using 50M of ram, wouldn't it better for it to
use only 25M when that's all it needs?  That's clearly a problem in my mind.
 Can you address why that is not a problem?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,283216,283465#msg-283465

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